yzf400 piston, valve replacement

mrjones

I have a 99 yzf400 that I bought in 2002. It is my first 4-stroke dirt bike. I've put over 125 hours on it, and have never replaced the top end, including the valves and springs. I've had the valves adjusted once. I need to know what parts need to be replaced in addition to the piston, valves, springs, and gaskets; like valve guides, cam chain. Also, what I need to look for and ask about when finding someone to do the work. Should they do a leakdown or compression check? Or a solvent test to check for leaking at the valves seat? Should they do this before dissassembly? Are they supposed to check the tolerances and specs of the parts connected with the top-end, like cam lobes, bores, etc., as part of the rebuild? Should they give me the replaced parts? Any ideas on the labor charge?

642MX

If it where mine, I would start by tearing it down and inspecting the top end. Its a given that you need a cam chain, piston, rings, and valve springs. I would carefully check the tensioner, chain guides, and valve buckets for wear. If the buckets have a bunch of circular marks on them, then they need replaced.

The cylinder needs to be measured for wear. The valves and seats need to be inspected for wear aswell. The 400 had SS valves, so they may still be good. You may want a machine shop to touch up the seats, and then relap the valves in.

I don't trust anybody to work on my bikes. I do all my own stuff. Anybody with a hand full of basic tools can do a 4 stroke top-end. The only specialty tools you need is a flywheel pull and a valve spring compressor. You can buy a flywheel puller for $20, but you may want a machine shop to do the cylinder head service for you.

mrjones

Hi, and thanks for the welcome! Thanks for the advice as well. I too am leery about having others do the work. You never know what you're going to get, unless you know the person doing the work. I have a shop manual, and would try to do it myself, but I don't have the tools like the spring compressor, and I'm a little leery about checking the chain tensioner. It doesn't look so easy. I've done a two-stroke top-end before, including removing and cleaning the valves, but this looks significantly harder.

Besides the main parts, like the piston, rings, chain, valves and springs, do you replace any other parts associated with the valve train, like the little retainers, valve keeps, and valve seats? Do you buy OEM parts? Also, do you know of machine shops that specialize in motorcycle engines, and/or would that matter?

642MX

Hi, and thanks for the welcome! Thanks for the advice as well. I too am leery about having others do the work. You never know what you're going to get, unless you know the person doing the work. I have a shop manual, and would try to do it myself, but I don't have the tools like the spring compressor, and I'm a little leery about checking the chain tensioner. It doesn't look so easy. I've done a two-stroke top-end before, including removing and cleaning the valves, but this looks significantly harder.

Besides the main parts, like the piston, rings, chain, valves and springs, do you replace any other parts associated with the valve train, like the little retainers, valve keeps, and valve seats? Do you buy OEM parts? Also, do you know of machine shops that specialize in motorcycle engines, and/or would that matter?

I reuse the keepers and retainer plates. I do buy OEM parts, all except the piston. I use a Wiseco piston in mine.

I can't recommend any engine builders, because I haven't had to use them. Maybe somebody else can chime in??